1. Field of the Invention
The present invention relates to an electrophotographic machine, and more particularly, to a laser printhead mounting apparatus and printhead skew adjustment mechanism for an electrophotographic machine, such as a laser printer.
2. Description of the Related Art
In a typical in-line color electrophotographic imaging process, latent images are formed on a plurality of photosensitive drums, which are in turn each developed using a predetermined color of toner. Typically, these colors are black, magenta, cyan and yellow. The developed images are then transferred to either an intermediate transfer medium or directly to a sheet of media (such as paper) that travels past the photosensitive drums. The image in each color is created one line at a time, and the lines are oriented at right angles to the direction of travel of the media. The individually-generated images combine to form a full-color image. Thus, in a typical multi-color laser printer, the sheet of media receives color images generated at each of the four image developing stations.
It is recognized that in order for the multi-color laser printer to print accurately, the laser beams for all four colors must be in alignment, both in the scan direction (i.e., the direction the laser sweeps across the photoreceptive medium) and the process direction (feed direction of the print medium). However, providing proper alignment of even a single laser printhead in relation to the sheet of media in the process direction can be difficult. This problem is compounded with the addition of each printhead, since the plurality of printheads must be in registration so that the individual images generated by each printhead can be superimposed correctly when combined. During printer assembly an attempt is made to optically align the laser printheads both individually and collectively, but the ability to provide precise alignment is limited by several factors, including component manufacturing tolerances. In addition, it is possible for a precisely aligned printhead to drift out of alignment over time due to component aging and ambient environmental factors, such as temperature. Skew is one such alignment parameter which can be corrected by mechanical rotation of the printhead relative to a pivot point located in the printer mounting frame. Skew is the slope of a least squares fit straight line through all of the laser spots across a scan line.
It is known that one can use one of a variety of sliding or pivoting mechanisms to mount a printhead to a printer frame and to provide adjustment of the position of a printhead in an adjustment direction to correct printhead skew. However, such sliding mechanisms are difficult to control when attempting to make small adjustments necessitating movement of the slide mechanism over a very short distance. In attempting small adjustments, there is a likelihood that the mechanism will exhibit the phenomenon know as "stick-slip", or frictional hysteresis, which makes repeatability uncertain. For example, an executed command to move the slide mechanism a certain distance at one time may not be repeatable in producing the same amount of motion at another time. In addition, when the fasteners mounting the printhead to the mounting frame are tightened, unwanted rotation can produce an error in skew registration.
What is needed in the art is an apparatus that can consistently provide precise and repeatable printhead skew adjustment in an electrophotographic machine to compensate for printhead alignment errors due to printhead skew.